1. Field of the Invention
The present invention relates to a liquid crystal display device, more particularly to a liquid crystal display device having a liquid crystal display panel that may decrease the failure of a tape automated bonding (TAB) by decreasing occasions of a printed circuit hoard (PCB) when a ductile printed circuit hoard (PCB) misalignment such as a tape carrier package (TCP) and the PCB are bonded by a thermo-compression bonding method.
2. Description of the Related Art
At present, a lot of display devices having minimized sizes and more powerful functions are manufactured as the semiconductor technology rapidly develops. The cathode ray tube (CRT) widely used for an information display device has some advantages such as high performance and low cost. However, the CRT has disadvantages of its bulky size and poor portability. The liquid crystal display (LCD) device has a smaller size and lighter weight. In addition, the LCD device can operate at low power. Hence, the LCD device has been paid much attention as substitute for the CRT and is now used for virtually all the information processing devices.
In general, the LCD device includes a thin film transistor (TFT) substrate, a color filter substrate opposed to the TFT substrate and a liquid crystal display panel having a liquid crystal injected between the TFT substrate and the color filter substrate. The LCD device displays the information by utilizing a light modulation resulting from the variation of the optical property of the liquid crystal.
A plurality of gate lines and a plurality of intersecting data lines are formed on the TFT substrate, and thin film transistors for switching device and the picture electrode are formed in the intersected region.
The data line receives the gray voltage selected by a source driving integrated circuit (IC), and then transfers the gray voltage to the liquid crystal. The gate line opens or closes the thin film transistor for switching according to the on/off signal outputted from a gate driving IC. At that time, in order to apply the driving signals to the gate line and the data line, the printed circuit board (PCB) including various semiconductor devices and parts thereon, the driving IC transferring the driving signals to the gate line and the data line and the liquid crystal display panel indicating the information should be connected one after another.
The methods for connecting the liquid crystal panel, the printed circuit board and the driving IC are generally divided into a chip on glass (COG) mounting method and a tape automated bonding (TAB) mounting method.
In the COG mounting method, the liquid crystal display panel is connected to the printed circuit board by using a connector such as a flexible printed circuit (FPC) after the driving IC is mounted on the liquid crystal display panel. Also, in the TAB mounting method, the liquid crystal display panel is connected to the printed circuit board by using a tape carrier package (TCP) including a tape and the driving IC mounted on the tape.
In the conventional TAB mounting method, the liquid crystal display panel is connected to the TCP by a thermo-Compression bonding process and by using an anisotropic conductive film (ACF), and the printed circuit board is connected to the TCP by a soldering process. However, the pitch of the input leads of the TCP must decrease as the number of the input leads of the TCP increases and the size of the TCP is reduced. As a consequence, the probability of short-circuits between the adjacent input leads of the TCP increases when the printed circuit board and the TCP are coupled to each other in the soldering process. Hence, the printed circuit board and the TCP are now being coupled together with a thermocompression bonding process.
FIG. 1 is a plan view illustrating the conventional liquid crystal display device including the printed circuit board and the tape carrier package bonded together by the thermo-compression bonding process at high temperature.
Referring to FIG. 1, a liquid crystal display panel 10 receives the electrical signal from the outside, and then displays the information thereon. Printed circuit boards 20 and 30 are connected to the liquid crystal display panel 10 and transfer the electrical signal to the liquid crystal display panel 10. Tape carrier packages 40 and 50 connect the liquid crystal display panel 10 to the printed circuit boards 20 and 30 to drive the liquid crystal display panel 10.
The liquid crystal display panel 10 includes a TFT substrate 14 and a color filter substrate 12 facing each other.
A plurality of gate lines (not shown) are disposed on the TFT substrate 14 along the length of the TFT substrate 14 and a plurality of data lines (not shown) are disposed on the TFT substrate 14 along the width of the TFT substrate 14. The gate lines and the data lines intersect each other. Gate input pads and data input pads (not shown) are respectively formed on each end of the gate lines and the data lines outside the color filter substrate 12.
The printed circuit boards 20 and 30 comprise a gate printed circuit board 20 electrically connected to the gate input pads through the TCPs 40, and a source printed circuit board 30 electrically connected to the data input pads through the TCPs 50.
A gate driving IC 42 and a source driving IC 52 that respectively drive the gate lines and the source lines are formed on the respective upper surfaces of the tape carrier packages 40 and 50.
Hereinafter, a method is described for connecting the liquid crystal display panel 10 to the printed circuit boards 20 and 30 by using the tape carrier packages 40 and 50.
At first, after the anisotropic conductive film is attached to the data pads and the gate pads on the TFT substrate, the output leads (not shown) of the tape carrier packages 40 and 50 are positioned on the surface of the anisotropic conductive film, and then the surfaces of the tape carrier packages 40 and 50 are pressed by a thereto-compression device. Thus, the gate pads and the data pads and the output leads of the tape carrier packages 40 and 50 are electrically connected while the anisotropic conductive film composed of a thermoplastic resin is completely compressed to the liquid crystal display panel 10 by the thermo-compression device.
Subsequently, after the anisotropic conductive film is attached to the surface of the respective PCB land groups (not shown) formed on the respective rear surfaces of the printed circuit boards 20 and 30, the input leads (not shown) of the tape carrier packages 70 and 90 are respectively attached to the land groups on the rear surfaces of the printed circuit boards 20 and 30 by using the thermo-compression device. Hence, the respective input leads of the tape carrier packages 40 and 50 are electrically connected to the respective PCB lands (not shown) of the printed circuit boards 20 and 30 while the anisotropic conductive film is hardened by the heat and the force of the thermo-compression device at a high temperature.
In the above-described method, however, thermal expansions of components may cause misalignments between the lands of the printed circuit boards 20 and 30 and the leads of the tape carrier packages 40 and 50, thereby causing bonding failures.
FIG. 2 is a plan view for showing the directions of thermal expansion of the printed circuit board and the tape carrier packages, as indicated therein by the respective arrows, during the thermo-compression bonding process. FIG. 2 shows the thermal expansions of the source printed circuit board and the tape carrier packages of FIG. 1.
As shown in FIG. 2, the printed circuit board 30 is thermally expanded from a central point M horizontally dividing the printed circuit board 30 into two equal portions disposed on opposite sides thereof. The amount of thermal expansion of the printed circuit board 30 is accumulated toward both opposite end portions of the printed circuit board 30 so that the two end portions have the greatest amount of thermal expansion. Also, each tape carrier package 50 is thermally expanded from a central point M′ horizontally dividing the tape carrier package 50 into two equal portions disposed on opposite sides thereof, and the two opposite end portions have the greatest amount of thermal expansion.
Thus, although the PCB lands group (not shown) of the printed circuit board 30 and the corresponding lead groups of the tape carrier packages 50 are aligned with each other before the thermocompression process, they get misaligned during the thermo-compression bonding process due to the thermal expansion between the printed circuit board 30 and the tape carrier packages 50. They are misaligned the most in the right portion (A1) of a first TCP and the left portion of an eighth TCP (A2).
In order to reduce the bonding failures between the PCB lands and the TCPs caused by the misalignment, the thickness of the printed circuit board may be increased, or the composition of the printed circuit board may be changed. However, those methods do not totally solve the misalignment problem when the pitches between the leads become smaller.